Field of the Invention
This invention relates to single cell and multiple cell foil packaged batteries and in particular to an expandable device for encasing a plurality of foil packaged batteries.
Relevant Background
Batteries come in many shapes and sizes. The most common type of battery is a cylindrical cell. Such a battery is easy to manufacture and offers a high energy density with good mechanical stability. The cylindrical cell, normally made of some type of metal, has the ability to withstand high internal pressure and is widely used in applications that do not demand an ultra-small size. Most cylindrical cells, however, are of a fixed size; thus the battery pack must be designed around established battery sizes.
Lithium ion batteries have generally been produced in cylindrical cells to provide a constant constraint and containment for the storage of electricity within a battery. Cylindrical cells have a limit to their capacity in amp hours. This limit greatly increases the number of cells needed to be attached together in order to provide large capacity storage of the lithium ion chemistry.
Button cells and prismatic cells were a natural progression of the cylindrical cell. However, button cells suffer from an inability to vent excess pressure that may exist in an overcharge situation and often require excessive time to recharge. Prismatic cells suffer from lower power density and higher manufacturing cost.
Pouch cells are the newest form of battery technology. In a pouch cell a flexible heat-sealable foil is used to enclose the battery components. Pouch cells can be made to exact design requirements therefore making efficient use of valuable space. With the absence of a metal enclosure the pack is significantly lighter. Pouch cells are used for lithium-based chemistries with a life cycle somewhat less than that of other battery designs.
When not constrained pouch cells expand and contract slightly with each charge and discharge cycle; and as the cell ages the pouch experiences a slight permanent expansion. For example, if a plurality of pouch cells is contained within a 10 inch region, by the time the pack of cells has experienced 1,000 cycles, 10.3 inches may be needed to house the same plurality of cells. To optimize performance and obtain maximum power density and cycle life, the electrodes within the pouches must be constrained under constant pressure. But, it is well known that non-flexible containment, or containment which causes internal cell pressure to increase during expansion, reduces cell performance. This reduction in performance is present when the cells are new and becomes more significant as the pouches age.
One pouch battery containment system known in the prior art uses neoprene or rubber material to surround the cells. Such a system is static, meaning that it can only provide the correct pressure on the pouch cells at one particular instance in the pouch's life cycle since as the cells age the containment system does not expand with the permanent expansion of the pouch cells. Thus the optimal capacity, performance and life cycle of the cells are curtailed.
A need therefore exists to contain pouch battery cells so as to place the electrodes within each pouch at a constant pressure during each charge/discharge cycle so as to optimize performance and life cycle. Because each pouch plastically deforms as it ages, a need also exists for a containment system that is adjustable to provide each pouch with constant pressure thereby optimizing performance over the pouch's entire life cycle. These and other improvements to the prior art are addressed by one or more embodiments of the present invention.